This invention relates generally to calculators and improvements therein and more particularly to non-programmable business calculators.
Owing to the high cost and the limited capabilities of the available business calculators, and sometimes, simply because there is no calculator available to perform certain calculations, the majority of the everyday business calculations are still made with the aid of published tables. Published tables are the only convenient means available for solving certain financial problems. The main disadvantage of using tables is the inherent restriction to the discrete values given in the table. The accuracy of the calculation is limited to the accuracy of the tables and the need for interpolation further compromises the calculation. For example, a widely used bond value table has discrete values for bond yield to two decimal places and the interest rate is given in one-eighth of one percent increments. The use of tables with this limited accuracy could lead to errors of several thousands of dollars in a fifty million dollar bond issue.
Conventional business calculators for bond price and bond yield calculations have a manual switch to initiate different bond price and bond yield algorithms for bonds maturing in less than 181 days (these are considered as notes rather than bonds). The present calculator has an automatic feature to check the maturity period and initiate the proper algorithm. The algorithms traditionally used in conventional business calculators to solve bond price and bond yield are very complex and require extensive hardware capability. This has made these calculators large, complicated, and expensive. In the present calculator, therefore, the complex bond price and bond yield problems (and the solutions thereof) was simplified to make either problem solvable using only five registers. This simplification provides an explicit term that eliminates the complexity of a series of summations, which would otherwise require substantially more hardware. Thus, two new algorithms requiring significantly less hardware for bond price and bond yield calculations are provided in a small, general purpose calculator at a fraction of a special bond calculator price.
The algorithms are stored in a read-only memory circuit including seven serial-address in, serial-instruction out read-only memories regulated by a control and timing circuit. This control and timing circuit includes a microprogrammed controller, which receives status conditions from throughout the calculator and sequentially outputs signals to control the flow of data. The control and timing circuit also scans the keyboard to obtain a six-bit read-only memory address, which is generated at the keyboard each time a key is actuated as required to initiate one or more algorithms for performing the functions associated with the actuated key.
Information from the addressed read-only memory is transmitted serially to an arithmetic and register circuit where a serial, binary-coded decimal (BCD) adder/subtractor performs the basic computations. The results of the computations are transmitted to registers in this circuit where they are either stored temporarily or outputted via a seven-segment, 15-digit, LED display.